Diffusion Study of Mg-based Alloys: An Experimental and Modeling Approach

Sazol Kumar Das and In-Ho Jung

Department of Mining and Materials Engineering, McGill University, Montreal, QC, Canada, H3A 0C5

Anisotropic impurity diffusion coefficients of Al, Zn, Y, and Gd wereinvestigated for the first time using diffusion couples with Mg singlecrystal.

Anisotropic impurity diffusion coefficients of D ┴ / D ║ = 1.0 ~ 1.3,which is similar to Mg self-diffusion.

Solute diffusion coefficients decreases with increasing temperatureand can be neglected at common materials processing temperature forMg-based alloys.

The Al grain boundary diffusion coefficient was determined withmisorientation by developing a new technique with CFE-SEM EDS andfound to be about two order of magnitude faster at 75° misorientationbetween adjacent Mg grains.

Phase boundaries of stoichiometric compounds and solubility limits ofsolid solutions in the Mg-Y and Mg-Gd systems were determinedbetween 703 and 803 K and the phase diagrams of these two systemswere re-optimized.

ABSTRACT

Diffusion of alloying elements is key to control high temperatureprocesses (annealing, rolling, extrusion, etc.).

However, there is no systematic diffusion study or database for Mg.

Mg has anisotropic hcp crystal structure (c/a ratio: 1.62).

Self-diffusion in Mg is anisotropic D ┴ / D ║ = 1.13-1.26 [1-3].

Diffusion of Ag, Cd, In, Sb also shows anisotropy in hcp Mg [4].

Al and Zn are the prime alloying elements in Mg.

RE (Y and Gd) are also known to improve the property of Mg alloys.

There is no experimental study for anisotropic diffusion of Al, Zn, Y, and Gd in hcp Mg.

PROBLEM STATEMENTS

Diffusion of Ag, Cd, In, Sb and Sn in hcp Mg [4].[1] P. G. Shewmon ,Trans. Metall. Soc. AIME 206, 918 (1956): Single crystal radiotracer sectioning.

[2] J. Combronde et al., Acta Metall. 19, 1393 (1971): Single crystal radiotracer sectioning.

[3] M. Mantina , PhD thesis. Penn State University, 199 (2008): First principles calculations.

[4] Combronde et al., Acta Metall. 20, 37 (1972): Single crystal radiotracer sectioning.

To determine the anisotropic impurity diffusion coefficients of Al, Zn,

Y, and Gd in hcp Mg using single crystal Mg diffusion couples.

To determine grain boundary diffusion coefficients of solutes with

adjacent grains misorientation angles using Mg polycrystalline

diffusion couples.

To develop a diffusivity (mobility) database for Mg alloys.

To develop ICME tools for the solidification and homogenizationprocesses of Mg alloy production.

OBJECTIVES

Experimental conditions:Mg/Al: Temperature: 638 – 693 K; Duration: 2 – 4 daysMg/Zn: Temperature: 553 – 593 K; Duration: 4 – 20 daysMg/Y: Temperature: 723 – 803 K; Duration: 2 – 5 daysMg/Gd: Temperature: 703 – 743 K; Duration: 2 – 7 days

Diffusion equation (Fick’s 2nd law):

Solution of diffusion equation:

APPROACH

Pulley

Drum

Lid

Upper

Winding

Melt

Baffle

Crystal

Lower

Winding

Sketch of vertical Bridgman method [5].

Schematic of special cutting jig.

Mg Single Crystal

Before

After

Gas flow meter

Thermocouple

Controller

Steel Tube

Al

Mg

Before

X

MgAfter

Atmosphere-controlled low-temperature furnace. Samples before and after diffusion annealing.

EBSD, XRD Special Cuting

PolishingClamping on a-

and c-axisDiffusion Annealing

Quenching Mounting

Modeling

x

CD

xt

C mm

i

m

)])(())([(

)(211112

1 j

i

j

iii

j

i

j

iii

j

i

j

i CCDDCCDDx

tCC

FESEM-EDS EPMA-WDS

Slow Cuting

RESULTS

RESULTS

SEM-BSE image of interface microstructures of Mg-X diffusion couples.

Matano PlaneMatano Plane

Solute (Al, Zn, Y, Gd) concentration profiles through diffusion couple interfaces (EPMA-WDS).

Mg24Y5 Mg2Y YMg

MgY

Mg5Gd Mg3Gd Mg2Gd GdMg

MgGd

Mg4Gd6

Mg17Al12 MgMg2Al3Al Mg2Zn11 MgZn2 Mg2Zn3 MgZn

Al profile along grain boundary in comparison with bulk profile in hcp Mg.

Anisotropic impurity diffusion coefficients of Al, Zn, Y and Gd in theHCP Mg matrix were experimentally determined for the first time.Diffusivities of Y and Gd are about one order of magnitude lower thanthose of Al, Zn, and Mg.

Grain boundary diffusion coefficients of Al are two order of magnitudehigher at around 75° misorientation between adjacent Mg grains.

Phase boundaries in the Mg-Y and Mg-Gd systems were determinedaccurately in the solid solutions and intermetallics, and phase diagramswere re-optimized.

SUMMARY

693 K for 2 days

773 K for 2 days

593 K for 4 days

743 K for 2 days

693 K for 2 days

773 K for 2 days

593 K for 4 days

743 K for 2 days

[5] Prof. Tae Kwon Ha’s Research laboratory, Gangneung-Wonju National University, South Korea.

500μm

GB2 G1 G2 GB1 GB3

67o 76o 36o

o o o

hcp Mg

intermetallic layer Al

EBSD map

SEM SE micrograph

New technique to determine grainboundary diffusion coefficients withadjacent grain misorientation anglesusing CFE-SEM EDS from diffusioncouple experiments.

Calculated phase diagrams of the Mg-Y and Mg-Gd systems with experimental data.

CALPHAD XLIIICentral South University, ChangshaJune 1-6, 2014Changsha, China

Polishing

Impurity diffusion coefficients of Al, Zn, Y, and Gd in comparison with Al grain boundary and Mg self-diffusion coefficients in hcp Mg.

Publications:

- S. K. Das and I. -H. Jung, “Effect of the Basal Plane Orientation on Al and Zn Diffusion in hcp Mg”, Materials Characterization, 2014 (accepted).

- S. K. Das, Y. B. Kang, T. K. Ha and I.-H. Jung, “Thermodynamic Modeling and Diffusion Kinetic Experiments of Binary Mg-Gd and Mg-Y Systems”, Acta Materialia, 71, 164 (2014).

- S. K. Das, N. Brodusch, R. Gauvin, I.-H. Jung, “Grain boundary diffusion of Al in Mg” Scripta Materialia, 80, 41 (2014).

- S. K. Das, Y. –M. Kim, T. K. Ha, R. Gauvin, and I. –H. Jung, "Anisotropic diffusion behavior of Al in Mg: Diffusion couple study using Mg single crystal" Metall. Mater. Trans. A. 44, 2539 (2013).

- S. K. Das, Y. –M. Kim, T. K. Ha and I. –H. Jung, “Investigation of anistropic diffusion behavior of Zn in hcp Mg and interdiffusion coefficient of intermediate phase in Mg-Zn system”, Calphad, 42,

51 (2013).